In the drive for higher integration and operating speeds in LSI devices, the pattern feature size is made drastically finer. Under the miniaturizing trend, the lithography has achieved formation of finer patterns by using a light source with a shorter wavelength and by a choice of a proper resist composition for the shorter wavelength. Predominant among others are positive photoresist compositions which are used as a single layer. These single-layer positive photoresist compositions are based on resins possessing a framework having resistance to dry etching with chlorine or fluorine gas plasma and provided with a resist mechanism that exposed areas become dissolvable. Typically, the resist composition is coated on a substrate to be processed (processable substrate, hereinafter) and exposed to a pattern of light, after which the exposed areas of the resist coating are dissolved to form a pattern. Then, the substrate can be processed by dry etching with the remaining resist pattern serving as an etching mask.
In an attempt to achieve a finer feature size, i.e., to reduce the pattern width with the thickness of a photoresist coating kept unchanged, the photoresist coating becomes low in resolution performance. If the photoresist coating is developed with a liquid developer to form a pattern, the so-called “aspect ratio” (depth/width) of the resist pattern becomes too high, resulting in pattern collapse. For this reason, the miniaturization is accompanied by a thickness reduction of the photoresist coating (thinner coating).
On the other hand, a method commonly used for the processing of a processable substrate is by processing a substrate by dry etching with the patterned photoresist film made an etching mask. Since a dry etching method capable of establishing a full etching selectivity between the photoresist film and the processable substrate is not available in practice, the resist film is also damaged during substrate processing. That is, the resist film breaks down during substrate processing, failing to transfer the resist pattern to the processable substrate faithfully. As the pattern feature size is reduced, resist materials are required to have higher resistance to dry etching.
With the progress of the exposure wavelength toward a shorter wavelength, the resin in resist compositions is required to have less light absorption at the exposure wavelength. In response to changes from i-line to KrF and to ArF, the resin has made a transition to novolac resins, polyhydroxystyrene and aliphatic polycyclic skeleton resins. Actually, the etching rate under the above-indicated dry etching conditions has been accelerated. Advanced photoresist compositions featuring a high resolution tend to be rather low in etching resistance. This suggests the inevitableness that a processable substrate is dry etched through a thinner photoresist coating having weaker etching resistance. It is urgently required to have the material and process suited in this processing stage.
One solution to these problems is a multilayer resist process. The process involves forming an intermediate film on a processable substrate, forming a photoresist film (resist overcoat film) thereon, wherein the intermediate film with different etching selectivity from the resist overcoat film intervenes between the resist overcoat film and the processable substrate, patterning the resist overcoat film, dry etching the intermediate film through the overcoat resist pattern as an etching mask for thereby transferring the pattern to the intermediate film, and dry etching the processable substrate through the intermediate film pattern as an etching mask for thereby transferring the pattern to the processable substrate.
Included in the multilayer resist process is a bi-layer resist process. One exemplary bilayer resist process uses a silicon-containing resin as the overcoat resist material and a novolac resin as the intermediate film (e.g., JP-A 6-95385). The silicon resin exhibits good resistance to reactive dry etching with an oxygen plasma, but is readily etched away with a fluorine gas plasma. On the other hand, the novolac resin is readily etched away by reactive dry etching with an oxygen gas plasma, but exhibits good resistance to dry etching with fluorine and chlorine gas plasmas. Thus, a novolac resin film is formed on a processable substrate as a resist intermediate film, and a silicon-containing resin is coated thereon as a resist overcoat film. Subsequently, the silicon-containing resist film is patterned by exposure to energy radiation and post-treatments including development. While the patterned silicon-containing resist film serves as an etching mask, reactive dry etching with an oxygen plasma is carried out for etching away a portion of the novolac resin where the resist pattern has been removed, thereby transferring the pattern to the novolac film. While the pattern transferred to the novolac film serves as an etching mask, the processable substrate is etched with a fluorine or chlorine gas plasma for transferring the pattern to the processable substrate.
In the pattern transfer by dry etching, a transfer pattern having a relatively good profile is obtained if the etching mask has a satisfactory etching resistance. Since problems like pattern collapse caused by such factors as friction by a developer during resist development are unlikely to occur, a pattern having a relatively high aspect ratio is produced. Therefore, even though a fine pattern could not be formed directly from a resist film of novolac resin having a thickness corresponding to the thickness of an intermediate film because of pattern collapse during development due to the aspect ratio problem, the use of the bi-layer resist process enables to produce a fine pattern of novolac resin having a sufficient thickness to serve as a mask for dry etching of the processable substrate.
Also included in the multilayer resist process is a tri-layer resist process which can use general resist compositions as used in the single-layer resist process. In the tri-layer resist process, for example, an organic film of novolac resin or the like is formed on a processable substrate as a resist undercoat film, a silicon-containing film is formed thereon as a resist intermediate film, and an ordinary organic photoresist film is formed thereon as a resist overcoat film. On dry etching with a fluorine gas plasma, the resist overcoat film of organic nature provides a satisfactory etching selectivity ratio relative to the silicon-containing resist intermediate film. Then, the resist pattern is transferred to the silicon-containing resist intermediate film by dry etching with a fluorine gas plasma. With this process, even on use of a resist composition which is difficult to form a pattern having a sufficient thickness to allow for direct processing of a processable substrate, or a resist composition which has insufficient dry etching resistance to allow for substrate processing, a pattern of novolac film having sufficient dry etching resistance to allow for substrate processing is obtainable like the bilayer resist process, as long as the pattern can be transferred to the silicon-containing film.
The silicon-containing resist intermediate films used in the tri-layer resist process described above include silicon-containing inorganic films deposited by CVD, such as SiO2 films (e.g., JP-A 7-183194) and SiON films (e.g., JP-A 7-181688); and films formed by spin coating, such as spin-on-glass (SOG) films (e.g., JP-A 5-291208, J. Appl. Polym. Sci., Vol. 88, 636-640 (2003)) and crosslinkable silsesquioxane films (e.g., JP-A 2005-520354). Polysilane films (e.g., JP-A 11-60735) would also be useful. Of these, the SiO2 and SiON films have a good function as a dry etching mask during dry etching of an underlying organic film, but require a special equipment for their deposition. By contrast, the SOG films, crosslinkable silsesquioxane films and polysilane films are believed high in process efficiency because they can be formed simply by spin coating and heating.
The applicable range of the multilayer resist process is not restricted to the attempt of increasing the maximum resolution of resist film. For example, in a via-first method which is one of substrate processing methods where an intermediate substrate to be processed has large steps, an attempt to form a pattern with a single resist film encounters problems like inaccurate focusing during resist exposure because of a substantial difference in resist film thickness. In such a case, steps are buried by a sacrificial film for flattening, after which a resist film is formed thereon and patterned. This situation entails inevitable use of the multilayer resist process mentioned above (e.g., JP-A 2004-349572).
While silicon-containing films are conventionally used in the multilayer resist process, they suffer from several problems. For example, as is well known in the art, where an attempt is made to form a resist pattern by photolithography, exposure light is reflected by the substrate and interferes with the incident light, incurring the problem of so-called standing waves. To produce a microscopic pattern of a resist film without edge roughness, an antireflective coating (ARC) must be provided as an intermediate layer. Reflection control is essential particularly under high-NA exposure conditions of the advanced lithography.
In the multilayer resist process, especially the process of forming a silicon-containing film as an intermediate layer by CVD, it becomes necessary for reflection control purposes to provide an organic antireflective coating between the resist overcoat film and the silicon-containing intermediate film. However, the provision of the organic ARC entails the necessity that the organic ARC be patterned with the resist overcoat film made a dry etching mask. That is, the organic ARC is dry etched with the resist overcoat film made a dry etching mask, after which the process proceeds to processing of the silicon-containing intermediate layer. Then the overcoat photoresist must bear an additional load of dry etching corresponding to the processing of the ARC. While photoresist films used in the advanced lithography become thinner, this dry etching load is not negligible. Therefore, greater attention is paid to the tri-layer resist process in which a light-absorbing silicon-containing film not creating such an etching load is applied as an intermediate film.
Known light-absorbing silicon-containing intermediate films include light-absorbing silicon-containing films of spin coating type. For example, JP-A 2005-15779 discloses the provision of an aromatic structure as the light-absorbing structure. Since the aromatic structure capable of effective light absorption acts to reduce the rate of dry etching with a fluorine gas plasma, this approach is disadvantageous for the purpose of dry etching the intermediate film without an additional load to the photoresist film. Since it is thus undesirable to incorporate a large amount of such light-absorbing substituent groups, the amount of incorporation must be limited to the minimum.
Further, the dry etching rate of the resist undercoat film during reactive dry etching with an oxygen gas plasma as commonly used in the processing of the resist undercoat film with the intermediate film made a dry etching mask is preferably low so as to increase the etching selectivity ratio between the intermediate film and the undercoat film. To this end, the intermediate film is desired to have a higher content of silicon which is highly reactive with fluorine etchant gas. The requirement arising from the conditions of processing both the overcoat or photoresist film and the undercoat or organic film gives preference to an intermediate film having a higher content of silicon which is highly reactive with fluorine gas.
In actual silicon-containing intermediate film-forming compositions of spin coating type, however, organic substituent groups are incorporated into the silicon-containing compounds so that the silicon-containing compounds may be dissolvable in organic solvents. Of the silicon-containing intermediate films known in the art, an SOG film-forming composition adapted for KrF excimer laser lithography is disclosed in J. Appl. Polym. Sci., Vol. 88, 636-640 (2003). However, since light-absorbing groups are described nowhere, it is believed that this composition forms a silicon-containing film without an antireflective function. This film fails to hold down reflection during exposure by the lithography using the advanced high-NA exposure system. It would be impossible to produce microscopic pattern features.
In addition to the dry etching properties and antireflection effect, the composition for forming an intermediate film with a high silicon content suffers from several problems, of which shelf stability is most outstanding. The shelf stability relates to the phenomenon that a composition comprising a silicon-containing compound changes its molecular weight during shelf storage as a result of condensation of silanol groups on the silicon-containing compound. Such molecular weight changes show up as film thickness variations and lithography performance variations. In particular, the lithography performance is sensitive, and so, even when the condensation of silanol groups within the molecule takes place merely to such an extent that it does not show up as a film thickness buildup or molecular weight change, it can be observed as variations of microscopic pattern features.
As is known in the art, such highly reactive silanol groups can be rendered relatively stable if they are kept in acidic conditions. See C. J. Brinker and G. W. Scherer, “Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing,” Academic Press, San Diego (1990). Further, addition of water improves the shelf stability as disclosed in J. Appl. Polym. Sci., Vol. 88, 636-640 (2003), JP-A 2004-157469 and JP-A 2004-191386. However, the silicon-containing compounds prepared by the methods of these patent publications are not inhibited completely from condensation reaction of silanol groups even when any of these means is taken. The silicon-containing compound in the composition slowly alters with the passage of time, and a silicon-containing film formed from such an altered composition changes in nature. Then the composition must be held in a refrigerated or frozen state just until use, and on use, be brought back to the service temperature (typically 23° C.) and be consumed quickly.